Merkel cell carcinoma (MCC) is a neuroectodermal, AIDS-related cancer suspected to have a viral etiology. MCC is 13 times more common than expected among AIDS patients and occurs in post-transplant and other immunosuppressed patient populations. While rare, it is the most aggressive skin cancer and only 50% of patients with advanced disease survive 9 months or longer. There are no early detection markers and it is frequently misdiagnosed with other small round cell tumors. We identified a novel polyomavirus in MCC using digital transcriptome subtraction (DTS). This agent, Merkel carcinoma polyomavirus (MCPyV), is most closely related to African green monkey lymphotropic polyomavirus (LPyV). Between 15-30% of persons have antibodies to LPyV, which has led investigators to speculate on the existence of a closely related but undiscovered human virus. MCPyV has a 5387 bp genome and expresses T antigen proteins in MCC tumors. Most MCC are positive for an integrated MCPyV that is readily detected by PCR and by Southern blotting. In contrast only 8-16% of control tissues harbor detectable viral genome. On Southern blotting, the majority of MCPyV positive tumors show a monoclonal somatic insertion pattern suggesting that the virus contributes to clonal cell expansion. We have developed reagents required to study this new virus including infected cell lines, specific antibodies and an overlapping peptide library for the VP1 protein. Preliminary studies suggest that MCPyV is a common but not ubiquitous infection that contributes to MCC when the virus mutates and integrates into the host genome. Identification of specific biomarkers for MCPyV infection is essential to determine MCPyV's role in human cancers. We propose to develop screening blood tests for MCPyV infection using established polyomavirus immunodiagnostic techniques as well as new immunodiagnostic methods. We will identify and characterize human antibody responses to MCPyV infection using cell-based IFA, MCPyV peptide mapping and virus-like particle ELISA with sera from well-defined populations. These assays will be vetted for cross-reactivity to known human polyomaviruses to ensure assay specificity. We also propose generating specific monoclonal antibodies to be used to detect viral antigens in tissue microarrays so that common tumors can be screened for infection. Expression of viral proteins can also be correlated with cellular protein biomarker expression to determine clinical course and responsiveness to treatment. This is the first proposal for funding to study a new human tumor virus found in our laboratory, MCPyV, likely to cause Merkel cell carcinoma. Merkel cell carcinoma is a cancer of the elderly, AIDS, transplant and other immunosuppressed persons. It has the highest overall mortality rate among skin cancers and there are no specific preventive measures or diagnostic tests for MCPyV infection. If funded, this proposal will develop accurate blood assays to measure antibody proteins marking MCPyV infection and to measure viral proteins in tissues. These are required to understand human infection with this virus and to develop effective vaccines and drugs against MCPyV.